1. Field of the Invention
The present invention relates to the field of colour television circuits and more particularly to chrominance decoding circuits in a colour television receiver using the PAL system or similar systems.
2. Discussion of the Background
In the aforementioned systems, the chrominance information contained in the composite video signal are modulated in amplitude and phase on a given frequency subcarrier. In the case of the PAL system, the frequency of the chrominance subcarrier is standardized at 4.43 MHz and the phase of one of the modulation vectors varies from .+-.90.degree. from one line to the next. Therefore, in order to obtain chrominance signals B-Y and R-Y, making it possible to reconstitute the colour information, it is necessary to demodulate the chrominance information contained in the composite video signal. This is conventionally carried out by demodulating, in a synchronous demodulator, the U or V-modulated chrominance signal by a signal of the same frequency as the chrominance subcarrier and having a given phase relationship with the transmitted signals. This signal is generally obtained at the output of a local oscillator having a frequency which is as near as possible to the subcarrier frequency, said oscillator being associated with a feedback loop on the transmitted signal.
Thus, very diagrammatically, the chrominance decoding circuit is in the form illustrated in FIG. 1. The latter shows a conventional chrominance decoding circuit of a PAL-DL system, which is the most widely used system in PAL television transmission. In this case, the composite video signal arrives at input I after passing through a filter which eliminates the frequencies too remote from that of the chrominance subcarrier. This signal is then transmitted into an automatic gain control circuit comprising a variable gain amplifier 1 associated with a gain control circuit 2 operating for the duration of a sync signal Sync corresponding to the duration of the presence of the burst signal and controlling the variable amplifier 1 via a filtering and maintaining means 3. The gain control circuit 2 receives the output of the variable amplifier on its first inputs E1 and E2, forms the product thereof and compares this signal with a reference signal Ref for the duration of the burst signal, as was stated hereinbefore. The output of variable amplifier 1 is connected to a first input of adders 4 and 5 and to the input of a line delay 6 ensuring that the signals at frequencies close to that of the chrominance subcarrier are delayed by one line. The output of the delay line 6 is transmitted to the second inputs of adders 4 and 5, with the output of the delay line 6 being fed through the inverter 7 before being sent to the second input of the adder 4. Thus, at the output of adder 4 is obtained the difference between the direct and delayed signals called signal V and at the output of adder 5 their sum which is called signal U. The outputs of adders 4 and 5 are demodulated in synchronous demodulators 8 and 9 which, at their outputs, supply chrominance signals R-Y and B-Y. The synchronous demodulator 9 receives a signal generated by a local oscillator controlled at 4.43 MHz with a certain phase called phase 0.degree. and demodulator 8 receives the same signal, but phase-displaced by .+-.90.degree., as a function of the lines.
The controlled local oscillator is constituted by a VCO-type oscillator 10, whose frequency is given by a crystal 11 and a feedback loop on the burst signal. The feedback loop is constituted by a phase comparator 12, followed by a filtering and maintaining means 13. Phase comparator 12 also receives a sync signal Sync which is present throughout the duration of the burst signal. The signal at the output of oscillator 11 is supplied directly to demodulator 9 and via a phase shifter 14 which alternately performs a phase shift of .+-.90.degree. on demodulator 8.
This type of circuit operates satisfactorily when oscillator 10 is a crystal oscillator. However, it has a certain number of disadvantages due to the presence of the crystal. Thus, the crystal is an expensive component, which cannot be integrated. Therefore, attempts are made to use a crystal-free VCO for obtaining the signal at the frequency of the chrominance subcarrier. However, particularly in the presence of noise, when using crystal-free oscillators a signal with an incorrect phase is obtained, which leads to a reduction in the amplitude of the chrominance signals B-Y and R-Y and causes visible faults on the picture.